Neurokinin receptors in the rabbit iris sphincter characterised by novel agonist ligands.

We have used novel selective agonist ligands to examine neurokinin receptors mediating the contractile response to tachykinins in the rabbit iris sphincter preparation in vitro. The selective NK-1 receptor agonist delta-amino valeryl-[L-Pro9,N-Me Leu10]SP-(7-11) (GR73632) and the NK-3 receptor-selective agonist succ-[Asp6,N-Me-Phe8] SP-(6-11) (senktide) were both very active (concentration range 0.032 pM-10 nM and 0.1 pM-32 nM respectively), and were 933 and 16.6 times more potent than substance P, respectively, in contracting the iris. In contrast, the NK-2 selective agonist [Lys3,Gly8-R-gamma-lactam,Leu9]NKA-(3-10) (GR64349) was active only at the highest concentrations tested (3.2 nM-32 microM), and had 0.054 the activity of substance P. The presence of several peptidase inhibitors was without effect on the concentration-response relationship to substance P, GR73632, GR64349 or senktide. Tachykinins differed in their offset kinetics. Responses to GR73632, GR64349 and senktide were rapid in offset (times to reach half maximal responses were 1.5, 1.1 and 5.1 min, respectively), whereas responses to substance P were very much more prolonged in duration (time to reach half maximal response was 35.3 min). These results suggest the presence of both NK-1 and NK-3 receptors mediating contraction of the rabbit iris sphincter preparation. In addition, differences in response offset kinetics seem not to be due to differences in peptide metabolism, and suggest a property of substance P not shared by the other tachykinins used in this study.